[jcheng]

Maybe this:

https://en.wikipedia.org/wiki/Simulation_heuristic

AFAIU, when something bad happens to us, we focus our regret on the smallest/nearest action that could have been undone to change the outcome. So in this case, the bad thing being that you wanted a piece and there are none left--it's natural to blame whomever ate the last piece, because it's easier to imagine an alternate reality where the last piece wasn't eaten, than an alternate reality with some earlier piece not being eaten and everybody subsequently encountering a cake with one additional piece.

[nindalf]

Same reason the dude who filmed the Rodney King incident was blamed for the LA Riots. People figured that if he hadn't filmed it, the riots wouldn't have happened.